In the hydraulic cylinder circuits for mobile machines known from the prior art, the retraction of hydraulic cylinders under a compressive load, e.g. by pressure-free lowering of the lifting arm, is implemented by means of a throttle control. In doing so, the potential energy associated with the load on the cylinder is converted into heat as a result of the restriction of the pressurised volume flow. This process leads to a disadvantageous destruction of the potential energy present. Furthermore, this conversion into heat also necessitates the disadvantageous application of additional cooling power within the machine.
A common embodiment of the hydraulic cylinders or hydraulic cylinder-piston devices in mobile machines is the differential cylinder. Where the term cylinder is used herein, this can, depending on the context, also be interpreted by a person skilled in the art as a complete cylinder-piston assembly.
When a differential cylinder of this kind is retracted with the help of a throttle control and compressive load, it must be ensured that a refilling of the rod side cylinder chamber is guaranteed. This can be achieved, on the one hand, by the delivery of a corresponding supply flow volume through the operating pumps or, alternatively, the necessary refilling of the rod side cylinder chambers can be carried out by returning the restricted volume flow. The return of the restricted volume flow results in a division of the volume flow exiting the base side of the differential cylinder according to the ratio of the areas of the rod side and base side of the hydraulic cylinder, whereby one portion of the volume flow is directed into the rod side chambers of the cylinder, and the other portion into the tank.
If it is desired to store the potential energy associated with the process of lowering the lifting cylinder, the objective would be to store the largest possible portion of the available energy. In the field of hydraulics, this would mean the largest possible oil quantity under the highest possible pressure. The hydraulic circuits known from the prior art in which the return of a portion of the base side volume flow into the rod side chambers of the hydraulic cylinder has been implemented reduce the volume flow that is available for storage.
Different solutions exist at present for storing the potential energy when lowering the boom of mobile hydraulic machines. Solutions are known from the prior art in which one of the two cylinders is used to store energy. These solutions use a positive displacement machine in a closed circuit to refill the rod side chambers of both cylinders with the return flow from the second cylinder. One disadvantage of such devices is the lack of exchange of oil on the base side of the hydraulic cylinder that is connected to the accumulator. The oil volume is only moved between the hydraulic accumulator and base side of the cylinder.
In other known devices, a hydraulic pump is used to guarantee refilling of the rod side chambers when retracting the cylinders. Refilling by applying hydraulic power in this manner does not constitute an energy efficient means of controlling the hydraulic consumer.
A method of taking up the potential energy of the boom by means of a gas filled cylinder is also known. Such devices require a gas cylinder to also be integrated into the machine, which means higher integration costs. Furthermore, the storage volume of the gas storage cylinder must be designed to accommodate the entire travel of the drive, even if the entire travel is not used during normal working operation.
Various devices are currently known for transferring the stored hydraulic energy, for example direct transfer of the stored energy to the fan circuit of the respective machine. Depending on the operating point of the fan circuit, it may be necessary to restrict the volume flow supply from the hydraulic accumulator to the fan circuit. This leads to throttling losses and consequently to a disadvantageous reduction in the quantity of reusable hydraulic energy.
Furthermore, a method of using the stored hydraulic energy to directly supply the operating pumps is known. This requires a switching means to connect the suction side of the operating pump either with the hydraulic tank or the hydraulic accumulator. When the pump is not supplied from the hydraulic accumulator, the valve causes pressure losses that can affect the suction pressure of the pump and thereby lead to unfavourable operating conditions. Moreover, it is necessary to provide cooling and filtering means between the hydraulic accumulator and the intake of the operating pump.
Also known from the prior art are devices in which the hydraulic energy derived from the potential energy of the working equipment or relevant cylinder can be distributed between two parallel paths. Firstly the energy can be transferred directly back into the drivetrain via a recovery motor, and secondly the energy can be directed to a hydraulic accumulator.
The circuits known from the prior art therefore exhibit three disadvantages:
The potential energy of the lifting/lowering process is destroyed by the action of restricting the flow and cannot be used for other processes.
The potential energy of the lifting/lowering process is put into the hydraulic system in the form of heat energy and must then be directed away again by means of suitable cooling devices. These processes are also energy consuming.
The splitting of the base side volume flow when the lifting cylinder is lowered leads to a reduction in the potential amount of storable energy.
Therefore it is the objective of the present invention to store the potential energy associated with the compressive load on the hydraulic cylinder while at the same time being able to provide the necessary oil quantity to refill the rod side chambers of the hydraulic cylinders.
This object is achieved by means of a device for recovering hydraulic energy in a machine having the characteristics of claim 1. Advantageous further developments are described in the subclaims. The present invention provides for a device comprising at least a first differential cylinder-piston assembly having a differential cylinder with a separate rod and base side, at least a second differential cylinder-piston assembly having a differential cylinder with a separate rod and base side, and at least one hydraulic accumulator that can be hydraulically connected to at least one of the differential cylinder-piston assemblies, wherein the differential cylinder-piston assemblies are mechanically coupled to one another, and wherein the potential energy of at least one or exactly one of the differential cylinder-piston assemblies retracting under a compressive load can at least partially be stored in the hydraulic accumulator.
Advantageously according to the invention, it is possible to maximise the quantity of storable potential energy that can be used for other tasks within the machine. Furthermore, the cooling power employed can be reduced since the cooling system within the machine has less dissipated heat to remove. As a result, the overall operation of the hydraulic machine can be more energy efficient.
Due to the coupling of at least two differential cylinder-piston assemblies, it is possible to store in the hydraulic accumulator the potential energy from one of the differential cylinder-piston assemblies when both differential cylinder-piston assemblies are retracted at the same time.
In a preferred embodiment, it is conceivable that the hydraulic accumulator can be hydraulically connected to more than one of the differential cylinder-piston assemblies. Accordingly, energy can be directed out of the hydraulic accumulator and into the respective differential cylinder-piston assemblies in the form of a pressurised hydraulic fluid.
In a further preferred embodiment, it is conceivable that an assisting motor is provided that is designed to direct into a drivetrain of the machine, and thereby recover, the hydraulic energy stored in the hydraulic accumulator, wherein the assisting motor can be hydraulically connected to the hydraulic accumulator in particular via a valve of the assisting motor. In a further preferred embodiment, it also conceivable that the device is designed to recover the energy stored in the hydraulic accumulator by transferring the energy to at least one of the differential cylinder-piston assemblies. This allows, advantageously, at least two types of energy recovery to be carried out, namely directly by returning the hydraulic fluid from the hydraulic accumulator to at least one differential cylinder-piston assembly, and alternatively or in addition to this via the assisting motor. The hydraulic energy directed from the hydraulic accumulator to the assisting motor can in this case be used, for example, to assist a primary drive motor of the machine.
In a further preferred embodiment, it is conceivable that in order to recover the hydraulic energy stored in the hydraulic accumulator, the hydraulic energy can be transferred either simultaneously, alternately or sequentially to the drivetrain and/or to at least one of the differential cylinder-piston assemblies. The transfer of energy in this case can, of course, occur by means of the transfer of a pressurised hydraulic fluid. This allows, advantageously, the energy stored in the hydraulic accumulator to be employed, depending on the current operating state of the machine, in the particular areas of the machine currently having the largest energy requirements, or where the recovery of the energy would have the greatest energy saving potential.
In a further preferred embodiment, it is conceivable that the differential cylinder-piston assemblies are arranged to operate in parallel. This enables two or more differential cylinder-piston assemblies to be provided, for example to pivot a joint of the machine.
In a further preferred embodiment, it is conceivable that at least one operating pump for driving the differential cylinder-piston assembly, and/or at least one slide valve for controlling the hydraulic accumulator and/or the differential cylinder-piston assembly, and/or at least one tank, and/or at least one hydraulic accumulator valve for shutting off the hydraulic accumulator is provided, and/or that each of the differential cylinder-piston assemblies is associated with at least one brake valve. The advantages of the individual components are evident from the description of the FIGURE.
In a further preferred embodiment, it is conceivable that a shut-off valve for isolating at least one of the differential cylinder-piston assemblies from the tank and from the operating pump is provided.
The present invention further relates to a machine, in particular a wheel loader, a hydraulic excavator or a crane having a device for recovering hydraulic energy in accordance with one of the claims 1 to 8, and in which, in a preferred embodiment, the machine is designed in such a manner as to be still operable without the loss of other functions in the event of a breakdown of the device for recovering hydraulic energy. The machine can be designed in such as way that the device for recovering hydraulic energy is arranged redundantly to the actuators provided on the machine.